Key messages

Two studies evaluated the effects of retaining existing bat roosts and access points within developments on bat populations. One study was in Ireland and one study was in the UK.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (0 STUDIES)

USAGE (2 STUDIES)

Use (2 studies): One before-and-after study in Ireland found similar numbers of brown long-eared bats roosting within an attic after existing access points were retained during renovations. One replicated, before-and-after study in the UK found that four of nine bat roosts retained within developments were used as maternity colonies, in two cases by similar or greater numbers of bats after development had taken place.

Background information and definitions

Many bat species are known to roost in the crevices and roof voids of buildings. Existing roosts and their access points may be conserved during residential or commercial developments, for example by retaining a roof space used as a roost during renovations.

For interventions that involve creating new bat roosts or relocating access points within developments, see ‘Create alternative bat roosts within developments’ and ‘Relocate access points to bat roosts within developments’.

Supporting evidence from individual studies

1

A before-and-after study in 2004–2008 of one building renovation in Ireland (Aughney 2008) found that retaining four existing bat access points, along with restricting the timing of roofing work, resulted in similar numbers of brown long-eared bats Plecotus auritus using a roost within an attic before and after renovations. Fifteen brown long-eared bats were counted roosting in the attic space of the building before the renovation work. After the renovation work, sixteen brown long-eared bats were recorded exiting the roost through the retained access points. The building was an 18th century Georgian house that had the roofing felt and roof slates replaced. Original access points to the roost within the attic of the building were retained by installing four vents in the ridge tiles. The renovations were completed outside of the maternity season (date not reported). The attic was surveyed once in 2004 before the renovations, and once with an emergence survey in September 2008 after the renovations.

2

A replicated, before-and-after study in 2011–2015 of nine bat maternity roosts retained within building developments across Scotland, UK (Mackintosh 2016) found that four of nine retained roosts were used by maternity colonies after development, and two of the roosts were used by greater or similar numbers of bats. Average roost counts before and after development at the four roosts either remained stable (before: 2 brown long-eared bats Plecotus auritus; after: 2 brown long-eared bats), increased by 7% (before: 476 soprano pipistrelles Pipistrellus pygmaeus; after 507 soprano pipistrelles), decreased by 39% (before: 341 soprano pipistrelles; after: 208 soprano pipistrelles), or could not be counted (use inferred from brown long-eared bat droppings only). The other five roosts were not used at all (two brown long-eared bat roosts, two common pipistrelle Pipistrellus pipistrellus roosts) or had signs of use by bats at a later date (one whiskered bat Myotis mystacinus roost). Original roosts were either retained (seven sites) or partially retained (two sites), and original access points were reinstated. The numbers of bats counted before development at each roost were extracted from reports submitted with licence applications. Bats were counted at each roost after development during at least one dusk emergence or dawn re-entry survey between May and September 2015.

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Effectiveness

An assessment by independent experts of the effectiveness of this action based on the summarized evidence (0% = not effective, 100% = highly effective). This score is based on the direction and size of the effects reported in each study. Actions with high scores typically have large, desirable effects on the target species/habitat in each study. There is some variation between actions, e.g. 100% effectiveness in adding underpasses under roads for bat conservation will likely have different impacts to 100% effectiveness in restoring marsh habitat. The effectiveness score does not consider the quantity or quality of studies; a single, poorly designed study could generate a high effectiveness score. The effectiveness score is combined with the certainty and harms scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Harms

An assessment by independent experts of the harms of this action to the target group of species/habitat, based on the summarized evidence (0% = none, 100% = major undesirable effects). Undesirable effects on other groups of species/habitats are not considered in this score. The harms score is combined with the effectiveness and certainty scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Certainty

An assessment by independent experts of the certainty of the evidence for this action based on the summarized evidence (0% = no evidence, 100% = high quality evidence). How certain can we be that the effectiveness score applies to all targets of the intervention (e.g. all birds for an action in the bird synopsis)? This score is based on the number, quality and coverage (species, habitats, geographical locations) of studies. Actions with high scores are supported by lots of well-designed studies with a broad coverage relative to the scope of the intervention. However, the definition of "lots" and "well-designed" will vary between interventions and synopses depending on the breadth of the subject. The certainty score is combined with the effectiveness and harms scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Overall Effectiveness Category

The overall effectiveness category is determined using effectiveness, certainty and harms scores generated by a structured assessment process with multiple rounds of anonymous scoring and commenting (a modified Delphi method). In this assessment, independent subject experts (listed for each synopsis) interpret the summarized evidence using standardised instructions. For more details see https://www.conservationevidence.com/content/page/79.